Particle classifier



April 15, 1969 D. K. DAVIS ET A1.` 3,438,264

PARTICLE CLASSIFIER Apr i115,1`9s9 D. K. DAVIS ETAL 3,438,264

PARTICLE CLASSIFIER Filed Aug. 2, 1967 sheet Z of s April 15, 1969 D K, DAvls ET AL PARTICLE CLASSIFIER 3 ofs Sheet Filed Aug. 2, 1967 Qms Opp, Je.

United States Patent O 3,438,264 PARTICLE CLASSIFIER Dearl Keith Davis, 2783 Fairlane Drive, Doraville, Ga.

30040; Warren P. Hendrix, Box 61, Atlanta Highway,

Lawrenceville, Ga. 30245; and Clyde Orr, Jr., 3281 Lenox Place, Atlanta, Ga. 30324 Filed Aug. 2, 1967, Ser. No. 657,902 Int. Cl. B07b 13/00 U.S. Cl. 73-432 11 Claims ABSTRACT OF THE DISCLOSURE A rotatable particle classifier has a classification chamber with a hollow slotted rotor for projecting particles into the chamber. Strip means positioned against the inner surface of the chamber receive the particles in a continuous spectrum of sizes. The strip means is removable for analysis of the particles distribution. A constant speed means rotates the rotor and chamber.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a particle classifier, and more partic-ularly to a particle classifier for classifying particles from subsieve to submicron sizes in a continuous spectrum of sizes.

Description of the prior art In the prior art, the classification of particles from subsieve to submicron sizes has been characteristically accomplished either on a cut size basis with particles of a size greater than the cut size being in one group and particles of a size less than the cut size being in another group or with the particles being separated into numerous SUMMARY OF THE INVENTION The invention disclosed herein solves these and other problems encountered in the prior art with respect to the classification of particles of subsieve to submicron sizes. This is because it provides for the classification of particles from subsieve to submicron sizes in a continuous spectrum of discrete sizes with a separation of particles by size which 'has not been achieved in the prior art.

These improvements in the classification of particles are accomplished in a particle classifier which includes a plurality of classification chambers formed by a divider wall positioned within a cylindrical dish, a rotor on which the plurality of classification chambers are positioned for rotation, a constant speed means for rotating the rotor and the plurality of classification chambers at a substantially constant predetermined speed regardless of temperature and other undesired changes in operating conditions, and feed means for feeding particles to be classified by size to a delivery aperture in each classification charnber. The rotation of the classification chambers at a substantially constant speed by a constant speed means causes particles entering each classification chamber to be distributed along a surface of a classification chamber in a continuous spectrum of discrete sizes by a centrifugal force field Iand provides a separation or classification of particles by size not achieved in the prior art.

3,438,264 Patented Apr. 15, 1969 A significant feature of the invention disclosed herein is the constant speed means since in its absence the substantially constant centrifugal force field required for good separation of particles by size is impossible. This is because variations in the centrifugal force field during the classification of particles by size would cause overlapping of particles having different sizes. Constant speed at several different values is also necessary to allow for variable particle density and size. The constant speed means is provided by an electrical motor for driving the rotor at a speed determined by the motor current, current means for varying the motor current to the motor in response -to the amplitude and duration of a control current, and substan` tially temperature stable control means for varying the amplitude and duration of the control current in response to changes in the speed of the motor.

BRIEF DESCRIPTION OF THE DRAWING These and other features and advantages of the invention will be more clearly understood from the following detailed description and the accompanying drawing in which:

FIG. 1 is a schematic presentation in section of the mechanical portion of a particle classifier embodying the invention disclosed herein;

FIG. 2 is taken in line 2-2 in FIG. l vand shows the plurality of classification chambers in the mechanical portion of that embodiment of the invention shown in FIG. 1;

FIG. 3 is a circuit diagram of the constant speed means provided b-y the electrical por-tion of a particle classifier embodying the invention disclosed herein.

DESCRIPTION OF AN EMBODIMENT The invention disclosed herein may be most easily understood as comprising a mechanical portion as shown in FIGS. l and 2 an-d an electrical portion as shown in FIG. 3, The mechanical portion serves to provide a means for distributing particles in a continuous spectrum of sizes along a wall of a classification chamber when the classification chamber is rotated at a substantially constant speed by the constant speed means provided by the electrical portion.

The mechanical portion is shown only schematically since details of its construction will be obvious to those skilled in the art and since it will be understood from the following description of an embodiment of the invention that it is the electrical portion of the invention which provides a particle classifier in which good separation or classification of particles in a continuous spectrum of sizes is achieved. However, although the electrical portion of the invention is required in combination with the mechanical portion in order for good classification of particles by size to be achieved, it should be understood that the electrical portion of the invention provides a constant speed means for many other applications which require a direct current motor having a substantially constant speed.

Mechanical portion Considering the mechanical portion of the invention, it will be seen from FIGS. 1 and 2, that a cylindrical dish A is divided by a wall B positioned along a diameter to form a plurality of classification chambers C and D. A cover E is positioned on the cylindrical dish A to close each of the classification chambers C and D. A rotor F with its centerline coinciding with the centerline of the cylindrical dish A extends through the cylindrical dish A and the cover E and divides the wall B into two equal segments B and B".

A transverse slot G extends through the rotor F in a direction transverse to the wall B to terminate in a def 3 livery aperture H communicatingwith the classification chamber C and in a delivery aperture H communicating with the classification chamber D. Above the transverse slot G the rotor F is hollow to provide a channel .l closed by a cap K through which extends a stationary feed tube L. At the lower end of the channel J above the transverse slot G is a ring insert M defining an orifice N. 1

The Wall O of the cylindrical dish A has a plurality of discharge apertures P and it will be understood that with rotation of the rotor F and the cylindrical dish A, a centrifugal force field is created which serves to discharge a fluid outwardly through the discharge apertures P and to pump the fiuid into the classification chambers C and D through the delivery apertures H and H', the slot G, the orifice N, the channel I, and the feed tube L. It,will be understood that rather than being in the wall O, a single or a plurality of apertures may be in the bottom of the cylindrical d-ish A or in the cover E as at P" so that they are not readily clogged with particles. However, with proper positioning of the apertures P or P", when particles to be classified by size are fed through a delivery line Q to the feed tube L as a well-dispersed aerosol powder from a device such as a conventional atomizer R, the particles are drawn into each of the classification chambers C and D by a pumping action which results from the centrifugal force field created by rotation of the cylindrical dish A.

It has been found that as particles enter the classificaion chambers C and D through the delivery apertures H and H', the centrifugal force field resulting from the rotation of the cylindrical dish A causes the particles to distribute themselves in a continuous spectrum of sizes along a strip S of plastic or similar material positioned against the inner surface of the wall O of the cylindrical dish A. v

After the particles to be classified have distributed themselves in a continuous spectrum of sizes along a strip S, the strip S may be removed for examination of the particles by microscope or for other uses. The strips S obtained from the plurality of classification chambers C and D should contain substantially identical distributions of particles and a plurality of classification chambers C and D are used to insure stability of the cylindrical dish A at rotational speeds up to approximately 12,000 r.p.m. However, regardless of the number of classification chambers C and D in an embodiment of the invention, the separation or classification by size of particles along a strip S is dependent upon maintaining a substantially constant speed of the cylindrical dish A while the particles are entering a classification chamber C or D. This is achieved by rotating the rotor F with a conventional gear arrangement T by a direct current electric motor by providing a constant speed means for maintaining the rotational speed of the motor 10 substantially constant as described in the following description of the electrical portion of the invention.

Electrical portion The electrical portion of the invention is shown in FIG. 3 and provides a constant speed means to insure that the motor 10 is driven at a substantially constant speed. In general, the electrical portion of the invention includes a conventional tachometer generator 11 attached directly to the shaft of the motor 10 so that the generator 11 is driven at =a speed directly related to the speed of the shaft of the motor 10 and the cylindrical dish A. The output signal from the generator 11 is amplified and used to vary the current to the motor 10 so that the motor 10 always maintains a substantially constant predetermined speed.

In considering the electrical portion of the invention in detail, it should be first noted that the motor 10 is connected by lines 12 and 26 across a standard AC line voltage through a switch S1 and a plurality of diodes arranged in a rectifier bridge configuration to convert an alternating current into :a direct current for the motor 10. It will be seen from FIG. 3 that with current fiow in the line 12 toward the motor 10, current flows from the line 12 through a diode 14, the motor 10, a diode 18, a wire 19, a diode 20, a silicon controlled rectifier 28, a diode 24, and to the line 26. With current flow in the line 26 toward the motor 10, current flows from the line 26 through a diode 22, the rectifier 28, a diode 21, the wire 19, a diode 16, the motor 10, a diode 15, and to the line 12. Thus, for both cycles of an AC line voltage :across the lines 12 and 26, current fiow through the motor 10 is in the same direction so as to provide a direct current. Moreover, this direct current is always through the rectifier 28 and control of the rectifier 28 as described below provides a means for controlling the speed of the motor 10 and the -cylindrical dish A. It will be understood that the motor 10 may be a universal type alternating current motor and the motor 10 connected directly to line 12 and Wire 19.

One side of the generator 11 is connected to a bus 30 and the other side is connected to the base of a transistor T1 through a pair of resistors 31 and 32 in series and through a pair of capacitors 34 and 35 in series and in parallel with the resistors 31 and 32. A point between the resistors 31 and 32 is connected through a capacitor 39 to the bus 30 and a point between the capacitors 34 and 35 is connected in parallel with the capacitor 39 through a resistor 41 to the bus 30. The base of the resistor 32 is connected to the bus 30` through a resistor 38 to minimize the effects of variations of leakage current in the transistor T1 and those skilled in the art will recognize the foregoing circuit arrangement including capacitors 34 and 35 as a conventional parallel T filter network.

The collector of' the transistor T1 is connected through a resistor 42 to a lead 44 and the emitter of the transistor T1 is connected through a resistor 45 to the bus 30. 'Phe transistor T1 is the 'first stage of a directly coupled amplifier,'the second stage of which includes a transistor T2. In this amplifier, the emitter voltage of the transistor T1 is applied directly to the emitter of the transistor T2. In addition, the collector voltage of the transistor T1 is divided between the resistors 51 and 46 and a portion applied to the base of the transistor T2 through resistor 49 in parallel with capacitor 54'. The collector and base of the transistor T2 are connected by a resistor 52 and capacitor 54 in parallel. Since the emitters of transistor T1 and transistor T2 are directly connected and since a portion of any voltage change resulting from a change in temperature of the transistor T1 is applied to the base of transistor T2 to change the voltage of the emitter of the transistor T1, the effects of a change in temperature of the transistors T1 and T2 on the amplification of the amplifier are nullified with proper selection of the resistors 51 and 46.

The arrangement of a resistor 52 and a capacitor 54 described above provides an output from the transistor T2 which is stabilized and more accurately related to the output of the generator 11. This is because the arrangement serves to reduce the effects due to thermal noise within the transistors T1 and T2, to reduce the effects of electrical noise from the commutator of the generator 11, and to provide negative feed back to increase the sta? bility of the transistor T2.

When a change of voltage is applied to the base of the transistor T2, a current change occurs in the emittercollector circuit for the transistor T2 which is between the lead 44 and the bus 30 and includes a resistor 55 in parallel with a capacitor 48, the transistor T2, and the resistor 45. In addition, the collector of the transistor T2 is connected through a resistor 59 to the base of a transistor T3. Thus, when current through the transistor T2 changes, a change in the collector potential of the transistor T2 is applied through a resistor 59 to the base of the transistor T3.

The emitter of the transistor T3 is connected to a Zener diode 161 which is connected between t-he lead 44 and the bus 30 in series with a resistor 65. Thercollector of the transistor T3 is connected through a resistor 62 to the emitter of a unijunction transistor T4. The Zener diode 61 and resistor 65 serve to provide a substantially constant reference voltage to the emitter of the transistor T3.

The emitter-collector circuit of the transistor T3 includes the resistor 62 and the emitter-collector circuit of a transistor T5. A capacitor 68 and diode 66 are in parallel between the emitter of the transistor T4 and the bus 30. Thus, with current flow through the transistor T3, the capacitor 68 is charged to apply a Voltage to the emitter of the unijunction transistor T4.

The transistor T4, the capacitor 68, and resistors 93 and 97 form a relaxation oscillator. A capacitor 69 is connected between the emitter of the transistor T4 and the base of the transistor T3 to provide AC coupled feed back from the collector to the base of the transistor T3. This makes the firing point of the relaxation oscillator more dependable.

The voltage applied to the base of the transistor T3 will adjust the frequency of the oscillator so as to 'vary the time at which the transistor T4 is triggered. However, it will be understood that the actual triggering voltage for the unijunction transistor T4 is supplied by the voltage on the capacitor I68 and that since the capacitor 68 is charged through the transistor T3 with a charging current related to the output from the generator 11, the voltage on the capacitor l68 will vary with the output signal from the generator 11. Since the voltage on the capacitor 68 determines when the unijunction transistor T4 conducts, the time of occurrence of voltage across the resistor 97 will vary with the output signal from the generator 11.

This voltage across the resistor 97 is connected through the gate circuit 72 of the rectifier 28 to the bus 30 and current flowing in the gate circuit 72 to the bus 30 determines the time at which the voltage between lines 12 and 26 start current to iiow through the rectifier 28. However, once the rectifier 28 is triggered by the required triggering voltage, current continues to flow through the rectifier 28 until voltage is substantially completely removed at the end of each half cycle of AC voltage.

Thus, as a pulsating rectified voltage is applied to the rectifier 28 'from the lines 12 and 26, the start of current iiow through rectifier 28 during each voltage pulse corresponding to a half cycle of alternating current voltage is dependent upon the time of firing of the unijunction transistor T4 as determined by the voltage applied to the emitter of the unijunction transistor T4. Therefore, current -iiowing through the rectifier 28 to the motor 10 is a pulsating direct current having a duration determined by the firing time of the transistor T4 and the voltage applied to the emitter of the transistor T4.

At the end of each pulse through the rectifier 28, the capacitor 68 is discharged so that the unijunction transistor T4 is fired only after and by the controlled charging current from the transistor T3. To insure that the capacitor 68 is discharged, the proper triggering voltage is placed on the base of the transistor T5 at the end of each pulse through the rectifier 28 so that the emittercollector circuit of the transistor TS discharges the capacitor 68 to a predetermined level.

For the unijunction transistor T4 to operate properly in controlling the rectifier 218 and the motor a constant voltage between the lead 44 and the bus 30A is provided by a circuit arrangement including a transformer l8f) having its primary winding 81 across the lines 12 and 26 and having its secondary winding `82 center tapped at 94 and arranged with four diodes 84, 85, 86, and 88 to provide a full-wave rectifier. One direction of current ow provided by the diodes 84, 85, 86, and 88 is from the center tap 94 through the diode 84, a resistor 91, a resistor 92, the bus 30 and to the cen'ter tap 94. The other direction of current flow is from the center tap 94 through the diode 85, the resistor 91, the resistor 92, the bus 30 and to the center tap 94. Thus, the transformer and the diodes 84, 85, 86, and 88 provide a rectified direct current voltage across the resistors 91 and 92 with the point being positive in potential.

`Connected to the point 901 through a resistor 96 is the base of a transistor T6. The emitter of the transistor T6 is connected to the base of a transistor T8 and the collector of the transistor T6 is connected to the point 90. The transistor T8 has its collector connected to the point 90`and its emitter connected to the lead 44.

A transistor T9 has its collector connected to the base of the transistor T6. The emitter of the transistor T9 is connected to the bus 30 through two Zener diodes 100 and .101 in series. The emitter of the transistor T8 and the lead 44 are connected in common to the bus 30 through resistors 87 and 89 in series and the emitter of the transistor T9 is connected in common with the emitter of the transistor T8 and the lead 44 through a resistor 97. A point between the resistors 91 and 92 is connected through a resistor 102 to a point 104 between resistors 87 and 89. This arrangement is a generally conventional series voltage regulator but in which the resistors 91, 92, and 102 provide a voltage to the base of transistor T9 that supplements the conventional voltage correction applied -from lead 44.

It will now lbe seen that when there is an increase in voltage from the transformer 80, the voltage across the transistor T9 is reduced to decrease the voltage applied to the base of the transistor T6. When the voltage applied to the base of transistor T6 decreases, the amplified voltage applied to the lead 44 is reduced. It will now also be seen that if there is a decrease in the voltage from the transformer 80, the voltage across the transistor T9 is increased to increase the voltage applied to the base of the transistor T6. When the voltage applied to the base of the transistor T6 increases, the amplified voltage applied to the lead 44 is increased. Thus, with the additional correction applied by the resistors 91, 92, and 102 provision is made for a highly stable voltage between the lead 44 and the bus 30.

A Voltage is also applied to the base of the transistor T 5 from the point 90" through a resistor .108 and a diode 108. Thus, during each half cycle of alternating voltage in the transformer 80, the transistor T5 is triggered to allow current to flow through its emitter-collector circuit to the capacitor 109 from the capacitor 68 to discharge the capacitor l68 to the level determined by the forward voltage of the diode 66. The capacitor 109 is charged during each half cycle from point 90 through resistor 109. The capacitor 103 is used in conventional manner to rreduce interface effects in the transistor T5.

It will now be understood that the constant speed means provides a substantially constant speed to the cylindrical dish A by varying the current to the motor 10 in response to changes in the speed of the generator 1.1 with a circuit arrangement which is highly stable in spite of the variations of temperature and other operating conditions. It will also be understood that this circuit arrangement includes means for varying the current to the motor 10 in response to the amplitude and duration of a control current, and substantially temperature stable means for varying the amplitude and duration of the control current in response to changes in the speed of the motor 10. Finally, it will be understood that the speed control means provided by the electrical portion of the invention in combination with the cylindrical dish A and a feed means for feeding particles to the cylindrical dish A provide a particle classifier in which the accurate separation and classification of particles in a continuous spectrum of sizes is achieved.

It will be obvious to those skilled in the art that many variations may be made in the embodiments chosen for the purpose of illustrating the present invention without departing from the scope thereof as defined by the appended claims.

We claim:

1. In a particle classifier for classifying particles by size by 'the application of a centrifugal force field, a hollow rotatable rotor having an axis of rotation, a substantially closed classification -chamber rotatable about said axis with said rotor and having a fluid discharge aperture therein, feed means for feeding particles to be classified by size into said classification chamber through a rdelivery aperture extending radially through the rotor wall, removable strip means positioned against the inner surface of said chamber, said particles being distributed in a continuous spectrum of sizes along said strip, and constant speed means for rotating said rotor at a substantially constant speed, said constant speed means including' amotor operativley connected to said rotor and having the characteristic of tending to increase its speed with an increase in motor current and to decrease its speed with a decrease in motor current, current means `for increasing and decreasing said motor current in response to the time of occurrence of a control cur-rent, and control means for changing the time of occurrence of said control current in response to changes in said speed of said motor.

2. The particle classifier of claim 1 in which said control means includes vol'tage means for providing a voltage output proportional to said speed of said motor, amplifying means for amplifying said voltage output, and current varying means for varying the time of occurrence of said control current in response to an output from said amplifying means.

3. The particle classifier of claim 2 in which said amplifying means includes a first transistor having a base to which said voltage output is applied and having an emitter operatively connected to the emitter of a second transistor and having a collector operatively connected to the base of said second transistor and said current varying means being responsive to changes in current flow through the emitter-collector circuit of said second transistor.

4. The particle classifier of claim 3 including a third transistor having a base to which a voltage proportional to said current ow is applied and having a capacitor charged by current through said third transistor, said current Varying means being responsive to the amplitude of a charge on said capacitor.

5. The particle classifier of claim 4 in which said current varying means is a fourth transistor having an emitter to which said charge is applied and having a circuit through which said control current passes.

6. The particle classifier of claim 5 including means for periodically discharging said capacitor.

7. The particle classifier of claim 5 in which said current means includes a switch means responsive to said control current for varying the time of occurrence of said motor current.

8. The particle classifier of claim 7 in which said switch means is a silicon controlled rectifier.

9. The particle classifier of claim 8 including constant voltage means for supplying a substantially constant voltage across the emitter-collector circuit of said second transistor.

10. The particle classier of claim 9 in which said constant voltage means includes a fifth transistor having an emitter-collector circuit responsive to changes in a line voltage and having a base to which a portion of said line voltage is directly applied.

11. The particle classifier of claim 1 in which said classification chamber is defined in part by a cover extending perpendicular to said axis and in which said discharge aperture is in said cover.

References Cited UNITED STATES PATENTS 2,956,434 10/ 1960 Donoghue 73-432 3,141,337 7/1964 Hoffstrom 73-432 3,234,447 2/1966 Sauber 3 l8-327 FRANK W. LUTTER, Primary xamner. 

